The postnatal development pattern of the ketone body-metabolizing enzymes in rat brain is unique among the development patterns of mitochondrial enzymes in this tissue. Also, activities of ketone body-metabolizing enzymes in developing rat brain are influenced by dietary and hormonal stimuli. These findings are solely based on the measurement of the enzyme activity per g tissue. The factors responsible for this unique postnatal developmental pattern of ketone body-metabolizing enzyme in rat brain are not understood. The aim of this proposal is to investigate, using 3-keto acid-CoA transferase as a representative of this group of enzymes in developing rat brain, the following: (i) the rates of synthesis and degradation of 3-keto acid-CoA transferae and the quantitation of the enzyme-mRNA in brains of normal and hypothyroid rats of varying ages from 1 day to 8 weeks, (ii) the influence of dietary modification (to cause hyperketonemia due to high fat diet) on the turnover of the enzyme, and (iii) the influence of ketone bodies and thyroid hormones on the synthesis of this enzyme in neuroblastoma and glioma cells in culture. To fulfill a prerequisite of having an antibody to the enzyme in question, we have purified to homogeneity 3-keto acid-CoA transferase from brains of one month-old rats, and raised the anitbody which precipitates the enzyme quantitatively. Using the standard experimental approach (i.e., in vivo labeling of the enzyme, immunoprecipitation of the labeled enzyme, separation on gel electrophoresis, etc.) the rates of synthesis and degradation of this enzyme will be measured in brains of growing rats. The changes in the concentration of the enzyme specific-mRNA in developing brain will be quantitated using an in vitro translation system. Neonatal hypothyroidism will be induced in one day-old rats by injecting 131I. Influence of the diet on the development of this enzyme in the brain will be monitored by changing dietary fat content available to suckling and weaned rats. To further illustrate and compare the effects of individual ketone body or thyroid hormones, the rates of synthesis of this enzyme in established neuroblastoma and glioma cells will be measured using the standard procedures.